Conventionally, in this type of flow meter device, a sing-around method is known, in which signal transmission and reception between two transducers is repeated plural times to enhance a measurement resolution.
An example in which this type of flow meter device is applied to a home gas meter will be described with reference to FIG. 11.
To be specific, in a fluid pipe (conduit) 101, a first transducer 102 which transmits an ultrasonic sound wave and a second transducer 103 which receives the ultrasonic sound wave are provided at an upstream side and a downstream side in a flow direction, respectively, and the ultrasonic sound wave travels across in an oblique direction a fluid flowing through the fluid pipe 101.
In addition, there are provided a measuring unit 104 for measuring a propagation time of the ultrasonic sound wave using the first and second transducers 102 and 103, a control unit 105 for controlling the measuring unit 104, and a calculating unit 106 for calculating a fluid flow based on a result of measurement of the measuring unit 104.
In FIG. 11, when a sound velocity is C, a flow velocity is v, a distance between the first and second transducers 102 and 103 is L, and an angle made between a propagation direction of the ultrasonic sound wave and a flow direction of the fluid is θ, a propagation time in a case where the first transducer 102 positioned at the upstream side on the fluid pipe 101 transmits the ultrasonic sound wave, and the second transducer 103 positioned at the downstream side on the fluid pipe 101 receives the ultrasonic sound wave is t1, and a propagation time of the ultrasonic sound wave in a reverse (opposite) direction is t2, t1 and t2 are calculated according to the following formulas:t1=L/(C+v cos θ)  (formula 1)t2=L/(C−v cos θ)  (formula 2)
(formula 1) and (formula 2) are changed into (formula 3) according to which the flow velocity v is derived.v=L·(1/t1−1/t2)/2 cos θ  (formula 3)
By multiplying a value derived from the (formula 3) by a value of a cross-sectional area of the fluid pipe, the fluid flow can be derived. In (formula 3), terms in parentheses can be changed as expressed as (formula 4):(t2−t1)/t1·t2  (formula 4)
A value of a denominator in (formula 4) is substantially constant irrespective of a change in the flow velocity, whereas a value of a numerator in (formula 4) is approximately proportional to the flow velocity.
Therefore, it is necessary to measure a difference between the two propagation times with accuracy. To this end, as the flow velocity is lower, it is necessary to derive a minute time difference. To perform measurement as a single-moment phenomenon, the measuring unit 104 is required to have a time resolution which is as small as ns (nano second) order.
It is difficult to implement the above time resolution. Even if the time resolution is implemented, electric power consumption increases due to the enhanced time resolution. Because of this, transmission of the ultrasonic sound wave is repeated in succession, and the measuring unit 104 measures a time required for the series of the repeated measurement.
By calculating an average value of the required value, a required time resolution is implemented. When the time resolution of the measuring unit 104 is TA, and the number of times of repeating is M, the measuring unit 104 operates continuously during the repeated measurement, and thus the measuring resolution of the propagation time is TA/M.
This type of flow meter device is capable of highly accurate measurement when a pressure in the fluid passage is stable. However, when this type of meter flow device is applied to a gas meter which measures a fluid flow of gas supplied to home as an energy resource, a unique problem called a pulsation arises.
The pulsation is a phenomenon which changes a pressure in a gas supply pipe in the vicinity of a gas engine in synchronization with rotation of the gas engine, like air-conditioning equipment using, for example, the gas engine called GHP. In the event of this pulsation, gas migrates within the pipe synchronously with a change in the pressure, even in a case where a gas instrument is not activated. Because of this migration, a measurement value of the fluid flow of the gas is detected, even though the gas is not flowing actually.
As a method of suppressing an influence due to the above phenomenon, for example, the number of times M of repeated measurement is lessened to a minimum number of times with which measuring accuracy can be ensured, measurement is conducted for a relatively long time N times in succession at short measurement intervals, and the fluid flow is calculated using results of measurement performed N times in succession.
Since the measurement intervals are set much shorter than a pressure change cycle, phases of a flow velocity change waveform can be captured evenly, and are averaged to effectively detect a genuine flow velocity (fluid flow) derived by excluding a change component (see e.g., Patent Literature 1).
Continuing the above measurement method all the time is not a good approach to electric power saving. To reduce unnecessary electric power consumption, the number of times N of measurement is controlled according to a change amount of the detected flow velocity. To be specific, under the situation in which a fluid flow change is small and it is determined that there is no pulsation, the number of times N of measurement is set smaller, while under the situation in which the fluid flow change is great and it is determined that there is a pulsation, the number of times N of measurement is set greater, (see e.g., Patent Literature 2).    Patent Literature 1 Japanese Laid-Open Patent Application Publication No. 2002-350202    Patent Literature 2 Japanese Laid-Open Patent Application Publication No. 2003-222548